Microwave sources are used for a variety of applications in which it is necessary to energize a material, for example in the formation of plasmas or ions for semiconductor processing, and as a heat source, for example in sintering ovens. However, the microwave sources typically employ a simple rectangular waveguide to deliver the microwave energy to the material processing region. Although that form of delivery may at times be relatively efficient, it does not uniformly energize the material, which is important when material processing uniformity is desirable. In addition, those microwave sources are typically not tunable to a wide range of load impedances, which may result in the inefficient use of the microwave energy.
For use with electron cyclotron resonance (ECR) plasma sources, the narrow impedance tuning range is a greater drawback. ECR sources typically have a much broader impedance range; the microwave sources currently available cannot be tuned to allow use under different ECR conditions. As a result, a number of interchangeable microwave sources must typically be used with ECR plasma sources, each of which is tunable to a small range of the ECR conditions.
Even in applications in which the load impedance can be closely matched, for example a plasma source with limited variation in gas composition, flow rate and pressure, the non-uniformity of the microwave field in the microwave sources results in very uneven materials processing. The non-uniform fields produce an equally non-uniform plasma, or non-uniform material heating, which causes variations in the materials processing parameters. In many applications, for example semiconductor substrate processing, those variations can greatly affect the product quality and yield.